Phase Relations and Chemical Composition of Phengite and Paragonite in Pelitic Schists During Decompression: a Case Study from the Monte Rosa Nappe and Camughera-Moncucco Unit, Western Alps |
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Authors: | KELLER L M; ABART R; SCHMID S M; DE CAPITANI C |
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Institution: | DEPARTMENT OF EARTH SCIENCES, BASEL UNIVERSITY, BERNOULLISTRASSE 32, CH-4056 BASEL |
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Abstract: | The metamorphic evolution of metapelites from the eastern partof the Monte Rosa nappe and the CamugheraMoncucco unit,both situated in the upper Penninic units SW of the Simplonline, were investigated using microstructural relationshipsand equilibrium phase diagrams. The units under considerationexperienced pre-Alpine amphibolite-facies conditions and underwenta complex metamorphic evolution during the Alpine orogeny. Peakpressures during an early Alpine high-pressure stage of 12·516kbar were similar in the Monte Rosa nappe and CamugheraMoncuccounit. A pronounced thermal gradient is indicated during decompressionleading to an amphibolite-facies overprint, as the decompressionpaths went through the chlorite, biotite and plagioclase stabilityfields in most of the Monte Rosa nappe, through the staurolitefield in the easternmost Monte Rosa nappe and in the CamugheraMoncuccounit, and through the sillimanite field in the easternmost CamugheraMoncuccounit. In high-Al metapelites the initial formation of stauroliteis related to continuous paragonite breakdown and associatedformation of biotite. In the course of this reaction phengitebecomes successively sodium enriched. In low-Al metapelites,in contrast, the initial staurolite formation occurs via thecontinuous breakdown of sodium-rich phengite. In both low- andhigh-Al metapelites the largest volume of staurolite is formedduring the continuous breakdown of sodium-rich phengite belowPT conditions of about 9·5 kbar at 600650°C.During this reaction phengite becomes successively potassiumenriched as sodium from phengite is used to form the albitecomponent in plagioclase. For normal pelitic chemistries,phengite becomes Na enriched during decompression through thebreakdown of paragonite along a near-isothermal decompressionpath. The Na content in phengite reaches its maximum when paragoniteis entirely consumed. During further decompression the paragonitecomponent in phengite decreases again because Na is preferentiallyincorporated into the albite component of plagioclase. KEY WORDS: metapelites; white mica; high pressure; equilibrium diagrams; Western Alps |
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Keywords: | : metapelites white mica high pressure equilibrium diagrams Western Alps |
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